Three-dimensional open architecture enabling salt-rejection solar evaporators with boosted water production efficiency

Direct solar desalination exhibits considerable potential for alleviating the global freshwater crisis. However, the prevention of salt accumulation while maintaining high water production remains an important challenge that limits its practical applications because the methods currently employed for achieving rapid salt backflow usually result in considerable heat loss. Herein, we fabricate a solar evaporator featuring vertically aligned mass transfer bridges for water transport and salt backflow. The 3D open architecture constructed using mass transfer bridges enables the evaporator to efficiently utilize the conductive heat that would otherwise be lost, significantly improving the water evaporation efficiency without compromising on salt rejection. The fabricated evaporator can treat salt water with more than 10% salinity. Moreover, it can continuously and steadily work in a real environment under natural sunlight with a practical solar-to-water collection efficiency of >40%. Using the discharged water from reverse osmosis plants and sea water from the Red Sea, the evaporator demonstrates a daily freshwater generation rate of ~5 L/m2, which is sufficient to satisfy individual drinking water requirements. With strong salt rejection, high energy efficiency, and simple scalability, the 3D evaporator has considerable promise for freshwater supply for water-stressed and off-grid communities.


REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): The paper entitled "Three-dimensional open architecture enabling salt-rejection solar evaporators with boosted water production efficiency" proposes a novel solution to limit salt crystal formation during solar water evaporation. This work brings very interesting research to this field and deserves to be published. The article is well written and I did not find any problem with the form. In my opinion the figures S1 S2 and S15 should be integrated in the article, otherwise it is difficult to understand what it is about.
Below are two remarks to improve the substance of the article: In the nighttime conditions one can clearly see the impact of the exchange area with the increase in the number of bridges. The problem of the study is that the performances are calculated in relation to the surface of the evaporator. However, increasing the number of bridges also increases the evaporator surface. In order to limit the effect of the exchange surface, it would have been interesting to estimate the evaporation performances for different heights of MTBs and for a variable number of MTBs in the case where the evaporator is enclosed. Such a study would increase the quality of the paper. On page 5, it is explained that the salt crystallizes at 14% concentration. What is the performance of the evaporator in this case. It would have been interesting to make a study according to the salinity of the water (e.g. 5, 10 15 20%) for several heights and several MTBs in a systematic way and for a case where the evaporator is closed. By creating 3D graphs, it would be possible to know under which conditions the performances degrade.
Reviewer #2 (Remarks to the Author): The manuscript 'Three-dimensional open architecture enabling salt-rejection solar evaporators with boosted water production efficiency' investigates vertically aligned mass transfer bridges in the solar evaporator to produce clean water. Although it is quite interesting, it's not ready to publish in the respected journal yet. Please refer to the following comments, which are helpful to the authors.
1. Line numbers 44 and 62: Although hydrophobic properties can prevent salt accumulation on the solar-absorber layer, a high concentration of saline water can penetrate that layer due to a low surface free energy of the high concentration of saline water. Additionally, if the authors emphasize the novelty, the authors should add some more data related to the contact angle of different NaCl concentration droplets on the absorber. 2. Line numbers 96 to 98: If this process is applied in a pilot or real scale levels, it takes more time than this experiment. Does this process has the feasibility to produce clean water at a commercial level? 3. Line numbers 120 to 121: A 12 h operation is insufficient to conduct occurring salt crystals. 4. Line numbers 131 to 133: The reviewer thinks that 1 g of NaCl is dissolved and penetrated to the opposite side due to a reverse salt flux due to the osmosis. The authors should confirm a mechanism to dissolve 1 g of NaCl. 5. Line numbers 137 to 139: It's related to the solubility of NaCl. A 14 wt% NaCl should be supersaturated on the absorber. 6. Line numbers 144 to 160: Although it seemed to be a significant experiment, it doesn't have a meaning in terms of feasibility. For example, if the authors change material instead of CNTs, it should be totally different. 7. Line number 232: It exhibited 5 L/m2/d, which means that it showed 0.56 L/m2/h. Is it possible to make a commercial level? The process is still insufficient to produce clean water instead of a commercial level of RO brine treatment processes such as membrane distillation in terms of feasibility. 8. Line numbers 250 to 251: If the evaporator will treat RO brine (e.g., RO brine of 1,000 t/d), how about the footprint of the evaporator? 9. Line numbers 277 to 278: Why are the authors applying partially oxidized CNTs on the GFM? Although it has a high conductive heat, the authors should explain it in the manuscript. Additionally, what is the difference between CNTs compared to the previous study? 1

Point-by-Point Responses to the Reviewers' Comments
Reviewer: 1 Comments: The paper entitled "Three-dimensional open architecture enabling salt-rejection solar evaporators with boosted water production efficiency" proposes a novel solution to limit salt crystal formation during solar water evaporation. This work brings very interesting research to this field and deserves to be published. The article is well written and I did not find any problem with the form. In my opinion the figures S1 S2 and S15 should be integrated in the article, otherwise it is difficult to understand what it is about.

Responses:
We are grateful to the reviewer for her/his appreciation of our work. Following the reviewer's suggestion, we have integrated Fig. S1, S2, S6 and S15 into Fig. 1, 2, 3 and 4 in the revised manuscript, respectively.

Comments:
In the nighttime conditions one can clearly see the impact of the exchange area with the increase in the number of bridges. The problem of the study is that the performances are calculated in relation to the surface of the evaporator. However, increasing the number of bridges also increases the evaporator surface. In order to limit the effect of the exchange surface, it would have been interesting to estimate the evaporation performances for different heights of MTBs and for a variable number of MTBs in the case where the evaporator is enclosed. Such a study would increase the quality of the paper.

Responses:
We thank the reviewer for this insightful comment. The reviewer is correct that the effect of exchange surface (i.e., the contribution of natural evaporation) can be largely eliminated by using an enclosed system for the measurement. In fact, the system we used for field tests was closed for achieving water collection. The reason we discuss open systems in the manuscript is for easier comparison with literature results on water evaporation (most previous works did not discuss water collection).
As suggested by the reviewer, we have added one paragraph to the revised manuscript discussing the effects of bridge number and bridge height on the water generation capacity of the enclosed evaporator (Fig. R1a). When the bridge height was fixed at 3 cm, the amount of collected water increased with the number of bridges (Fig. R1b). This result can be attributed to the alleviated salt accumulation and the decrease in salt concentration on the evaporating surface. When the bridge number was fixed at 32, the amount of collected water increased with the bridge height and reached the maximum at 3 cm, while further increasing the bridge height did not produce more water (Fig. R1c). This result is consistent with the conclusion above that 3 cm is sufficient to confine the conductive heat while further increasing bridge height only increases natural evaporation that has no effect on water 2 collection. In the revised manuscript, the new data for the enclosed system are shown in Fig.  S13 and discussed in the main text. Comments: On page 5, it is explained that the salt crystallizes at 14% concentration. What is the performance of the evaporator in this case? It would have been interesting to make a study according to the salinity of the water (e.g. 5, 10 15 20%) for several heights and several MTBs in a systematic way and for a case where the evaporator is closed. By creating 3D graphs, it would be possible to know under which conditions the performances degrade.

Responses:
We thank the reviewer for raising this point. Following the reviewer's suggestion, we discuss the water generation performance of an enclosed system under different salinity conditions in the revised manuscript. Given that the effects of bridge number and bridge height have been thoroughly investigated, we used the optimized conditions (i.e., 32 bridges; 3 cm high) to study water generation from brines with different salinities (wt% of NaCl).
The results showed that the water production efficiency monotonically decreased from ~0.73 kg/m 2 /h for 3.5 wt% brine to ~0.63 kg/m 2 /h for 20 wt% brine (Fig. R2a). The relatively low water production efficiency associated with the high-salinity brines is mainly due to their low saturated vapor pressure, partly due to the decreased photothermic conversion efficiency caused by salt precipitation. For instance, when using brine containing 20 wt% NaCl, salt precipitation was observed at the periphery of the evaporator after three hours of testing (Fig.  R2b). These results are shown in Fig. S14 and discussed in the main text in the revised manuscript.

Reviewer: 2
Comments: The manuscript 'Three-dimensional open architecture enabling salt-rejection solar evaporators with boosted water production efficiency' investigates vertically aligned mass transfer bridges in the solar evaporator to produce clean water. Although it is quite interesting, it's not ready to publish in the respected journal yet. Please refer to the following comments, which are helpful to the authors.

Responses:
We sincerely thank the reviewer for the insightful comments that indeed helped improve the quality of the manuscript.
Comments: Line numbers 44 and 62: Although hydrophobic properties can prevent salt accumulation on the solar-absorber layer, a high concentration of saline water can penetrate that layer due to a low surface free energy of the high concentration of saline water. Additionally, if the authors emphasize the novelty, the authors should add some more data related to the contact angle of different NaCl concentration droplets on the absorber.

Responses:
We agree with the reviewer that "a high concentration of saline water can penetrate the hydrophobic layer due to a low surface free energy". However, the reviewer might have some misunderstandings about our work. In our study, instead of using a hydrophobic layer to prevent brine penetration, we intentionally chose hydrophilic glass fiber membranes (i.e., MTBs) to connect the brine and the evaporator surface. Because the MTBs have highly hydrophilic channels, they can absorb brine immediately upon contact. Therefore, no contact angle analysis has been conducted. In our system, "salt rejection" is achieved by salt backflow via the MTBs, not by using a hydrophobic layer.
Comments: Line numbers 96 to 98: If this process is applied in a pilot or real scale levels, it takes more time than this experiment. Does this process has the feasibility to produce clean water at a commercial level?

Responses:
We thank the reviewer for raising this important concern. Our system can operate at larger scales by assembling the evaporator units into lateral arrays (see Fig. R3). Since the height of MTBs does not change, the time required to deliver brines to the evaporating surface will not change. Figure R3. Schematic illustration of module construction for large-scale applications.